src/random-seed/random-seed.c

   1 /* SPDX-License-Identifier: LGPL-2.1-or-later */
   2
   3 #include <errno.h>
   4 #include <fcntl.h>
   5 #include <getopt.h>
   6 #include <linux/random.h>
   7 #include <sys/ioctl.h>
   8 #if USE_SYS_RANDOM_H
   9 #  include <sys/random.h>
  10 #endif
  11 #include <sys/stat.h>
  12 #include <sys/xattr.h>
  13 #include <unistd.h>
  14
  15 #include "sd-id128.h"
  16
  17 #include "alloc-util.h"
  18 #include "fd-util.h"
  19 #include "fs-util.h"
  20 #include "io-util.h"
  21 #include "log.h"
  22 #include "main-func.h"
  23 #include "missing_random.h"
  24 #include "missing_syscall.h"
  25 #include "mkdir.h"
  26 #include "parse-argument.h"
  27 #include "parse-util.h"
  28 #include "pretty-print.h"
  29 #include "random-util.h"
  30 #include "string-table.h"
  31 #include "string-util.h"
  32 #include "strv.h"
  33 #include "sync-util.h"
  34 #include "sha256.h"
  35 #include "terminal-util.h"
  36 #include "util.h"
  37 #include "xattr-util.h"
  38
  39 typedef enum SeedAction {
  40         ACTION_LOAD,
  41         ACTION_SAVE,
  42         _ACTION_MAX,
  43         _ACTION_INVALID = -EINVAL,
  44 } SeedAction;
  45
  46 typedef enum CreditEntropy {
  47         CREDIT_ENTROPY_NO_WAY,
  48         CREDIT_ENTROPY_YES_PLEASE,
  49         CREDIT_ENTROPY_YES_FORCED,
  50 } CreditEntropy;
  51
  52 static SeedAction arg_action = _ACTION_INVALID;
  53
  54 static CreditEntropy may_credit(int seed_fd) {
  55         _cleanup_free_ char *creditable = NULL;
  56         const char *e;
  57         int r;
  58
  59         assert(seed_fd >= 0);
  60
  61         e = getenv("SYSTEMD_RANDOM_SEED_CREDIT");
  62         if (!e) {
  63                 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is not set, not crediting entropy.");
  64                 return CREDIT_ENTROPY_NO_WAY;
  65         }
  66         if (streq(e, "force")) {
  67                 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is set to 'force', crediting entropy.");
  68                 return CREDIT_ENTROPY_YES_FORCED;
  69         }
  70
  71         r = parse_boolean(e);
  72         if (r <= 0) {
  73                 if (r < 0)
  74                         log_warning_errno(r, "Failed to parse $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy: %m");
  75                 else
  76                         log_debug("Crediting entropy is turned off via $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy.");
  77
  78                 return CREDIT_ENTROPY_NO_WAY;
  79         }
  80
  81         /* Determine if the file is marked as creditable */
  82         r = fgetxattr_malloc(seed_fd, "user.random-seed-creditable", &creditable);
  83         if (r < 0) {
  84                 if (ERRNO_IS_XATTR_ABSENT(r))
  85                         log_debug_errno(r, "Seed file is not marked as creditable, not crediting.");
  86                 else
  87                         log_warning_errno(r, "Failed to read extended attribute, ignoring: %m");
  88
  89                 return CREDIT_ENTROPY_NO_WAY;
  90         }
  91
  92         r = parse_boolean(creditable);
  93         if (r <= 0) {
  94                 if (r < 0)
  95                         log_warning_errno(r, "Failed to parse user.random-seed-creditable extended attribute, ignoring: %s", creditable);
  96                 else
  97                         log_debug("Seed file is marked as not creditable, not crediting.");
  98
  99                 return CREDIT_ENTROPY_NO_WAY;
 100         }
 101
 102         /* Don't credit the random seed if we are in first-boot mode, because we are supposed to start from
 103          * scratch. This is a safety precaution for cases where we people ship "golden" images with empty
 104          * /etc but populated /var that contains a random seed. */
 105         r = RET_NERRNO(access("/run/systemd/first-boot", F_OK));
 106         if (r == -ENOENT)
 107                 /* All is good, we are not in first-boot mode. */
 108                 return CREDIT_ENTROPY_YES_PLEASE;
 109         if (r < 0) {
 110                 log_warning_errno(r, "Failed to check whether we are in first-boot mode, not crediting entropy: %m");
 111                 return CREDIT_ENTROPY_NO_WAY;
 112         }
 113
 114         log_debug("Not crediting entropy, since booted in first-boot mode.");
 115         return CREDIT_ENTROPY_NO_WAY;
 116 }
 117
 118 static int random_seed_size(int seed_fd, size_t *ret_size) {
 119         struct stat st;
 120
 121         assert(ret_size);
 122         assert(seed_fd >= 0);
 123
 124         if (fstat(seed_fd, &st) < 0)
 125                 return log_error_errno(errno, "Failed to stat() seed file " RANDOM_SEED ": %m");
 126
 127         /* If the seed file is larger than what the kernel expects, then honour the existing size and
 128          * save/restore as much as it says */
 129
 130         *ret_size = CLAMP((uint64_t)st.st_size, random_pool_size(), RANDOM_POOL_SIZE_MAX);
 131         return 0;
 132 }
 133
 134 static int load_seed_file(
 135                 int seed_fd,
 136                 int urandom_fd,
 137                 size_t seed_size,
 138                 struct sha256_ctx **ret_hash_state) {
 139
 140         _cleanup_free_ void *buf = NULL;
 141         CreditEntropy lets_credit;
 142         sd_id128_t mid;
 143         ssize_t k;
 144         int r;
 145
 146         assert(seed_fd >= 0);
 147         assert(urandom_fd >= 0);
 148
 149         /* First, let's write the machine ID into /dev/urandom, not crediting entropy. Why? As an extra
 150          * protection against "golden images" that are put together sloppily, i.e. images which are
 151          * duplicated on multiple systems but where the random seed file is not properly reset. Frequently
 152          * the machine ID is properly reset on those systems however (simply because it's easier to notice,
 153          * if it isn't due to address clashes and so on, while random seed equivalence is generally not
 154          * noticed easily), hence let's simply write the machined ID into the random pool too. */
 155         r = sd_id128_get_machine(&mid);
 156         if (r < 0)
 157                 log_debug_errno(r, "Failed to get machine ID, ignoring: %m");
 158         else {
 159                 r = random_write_entropy(urandom_fd, &mid, sizeof(mid), /* credit= */ false);
 160                 if (r < 0)
 161                         log_debug_errno(r, "Failed to write machine ID to /dev/urandom, ignoring: %m");
 162         }
 163
 164         buf = malloc(seed_size);
 165         if (!buf)
 166                 return log_oom();
 167
 168         k = loop_read(seed_fd, buf, seed_size, false);
 169         if (k < 0) {
 170                 log_warning_errno(k, "Failed to read seed from " RANDOM_SEED ": %m");
 171                 return 0;
 172         }
 173         if (k == 0) {
 174                 log_debug("Seed file " RANDOM_SEED " not yet initialized, proceeding.");
 175                 return 0;
 176         }
 177
 178         /* If we're going to later write out a seed file, initialize a hash state with the contents of the
 179          * seed file we just read, so that the new one can't regress in entropy. */
 180         if (ret_hash_state) {
 181                 struct sha256_ctx *hash_state;
 182
 183                 hash_state = malloc(sizeof(struct sha256_ctx));
 184                 if (!hash_state)
 185                         return log_oom();
 186
 187                 sha256_init_ctx(hash_state);
 188                 sha256_process_bytes(&k, sizeof(k), hash_state); /* Hash length to distinguish from new seed. */
 189                 sha256_process_bytes(buf, k, hash_state);
 190
 191                 *ret_hash_state = hash_state;
 192         }
 193
 194         (void) lseek(seed_fd, 0, SEEK_SET);
 195
 196         lets_credit = may_credit(seed_fd);
 197
 198         /* Before we credit or use the entropy, let's make sure to securely drop the creditable xattr from
 199          * the file, so that we never credit the same random seed again. Note that further down we'll write a
 200          * new seed again, and likely mark it as credible again, hence this is just paranoia to close the
 201          * short time window between the time we upload the random seed into the kernel and download the new
 202          * one from it. */
 203
 204         if (fremovexattr(seed_fd, "user.random-seed-creditable") < 0) {
 205                 if (!ERRNO_IS_XATTR_ABSENT(errno))
 206                         log_warning_errno(errno, "Failed to remove extended attribute, ignoring: %m");
 207
 208                 /* Otherwise, there was no creditable flag set, which is OK. */
 209         } else {
 210                 r = fsync_full(seed_fd);
 211                 if (r < 0) {
 212                         log_warning_errno(r, "Failed to synchronize seed to disk, not crediting entropy: %m");
 213
 214                         if (lets_credit == CREDIT_ENTROPY_YES_PLEASE)
 215                                 lets_credit = CREDIT_ENTROPY_NO_WAY;
 216                 }
 217         }
 218
 219         r = random_write_entropy(urandom_fd, buf, k,
 220                                  IN_SET(lets_credit, CREDIT_ENTROPY_YES_PLEASE, CREDIT_ENTROPY_YES_FORCED));
 221         if (r < 0)
 222                 log_warning_errno(r, "Failed to write seed to /dev/urandom: %m");
 223
 224         return 0;
 225 }
 226
 227 static int save_seed_file(
 228                 int seed_fd,
 229                 int urandom_fd,
 230                 size_t seed_size,
 231                 bool synchronous,
 232                 struct sha256_ctx *hash_state) {
 233
 234         _cleanup_free_ void *buf = NULL;
 235         bool getrandom_worked = false;
 236         ssize_t k, l;
 237         int r;
 238
 239         assert(seed_fd >= 0);
 240         assert(urandom_fd >= 0);
 241
 242         /* This is just a safety measure. Given that we are root and most likely created the file ourselves
 243          * the mode and owner should be correct anyway. */
 244         r = fchmod_and_chown(seed_fd, 0600, 0, 0);
 245         if (r < 0)
 246                 return log_error_errno(r, "Failed to adjust seed file ownership and access mode: %m");
 247
 248         buf = malloc(seed_size);
 249         if (!buf)
 250                 return log_oom();
 251
 252         k = getrandom(buf, seed_size, GRND_NONBLOCK);
 253         if (k < 0 && errno == EAGAIN && synchronous) {
 254                 /* If we're asked to make ourselves a barrier for proper initialization of the random pool
 255                  * make this whole job synchronous by asking getrandom() to wait until the requested number
 256                  * of random bytes is available. */
 257                 log_notice("Kernel entropy pool is not initialized yet, waiting until it is.");
 258                 k = getrandom(buf, seed_size, 0);
 259         }
 260         if (k < 0)
 261                 log_debug_errno(errno, "Failed to read random data with getrandom(), falling back to /dev/urandom: %m");
 262         else if ((size_t) k < seed_size)
 263                 log_debug("Short read from getrandom(), falling back to /dev/urandom.");
 264         else
 265                 getrandom_worked = true;
 266
 267         if (!getrandom_worked) {
 268                 /* Retry with classic /dev/urandom */
 269                 k = loop_read(urandom_fd, buf, seed_size, false);
 270                 if (k < 0)
 271                         return log_error_errno(k, "Failed to read new seed from /dev/urandom: %m");
 272                 if (k == 0)
 273                         return log_error_errno(SYNTHETIC_ERRNO(EIO), "Got EOF while reading from /dev/urandom.");
 274         }
 275
 276         /* If we previously read in a seed file, then hash the new seed into the old one, and replace the
 277          * last 32 bytes of the seed with the hash output, so that the new seed file can't regress in
 278          * entropy. */
 279         if (hash_state) {
 280                 uint8_t hash[SHA256_DIGEST_SIZE];
 281
 282                 sha256_process_bytes(&k, sizeof(k), hash_state); /* Hash length to distinguish from old seed. */
 283                 sha256_process_bytes(buf, k, hash_state);
 284                 sha256_finish_ctx(hash_state, hash);
 285                 l = MIN((size_t)k, sizeof(hash));
 286                 memcpy((uint8_t *)buf + k - l, hash, l);
 287         }
 288
 289         r = loop_write(seed_fd, buf, (size_t) k, false);
 290         if (r < 0)
 291                 return log_error_errno(r, "Failed to write new random seed file: %m");
 292
 293         if (ftruncate(seed_fd, k) < 0)
 294                 return log_error_errno(r, "Failed to truncate random seed file: %m");
 295
 296         r = fsync_full(seed_fd);
 297         if (r < 0)
 298                 return log_error_errno(r, "Failed to synchronize seed file: %m");
 299
 300         /* If we got this random seed data from getrandom() the data is suitable for crediting entropy later
 301          * on. Let's keep that in mind by setting an extended attribute. on the file */
 302         if (getrandom_worked)
 303                 if (fsetxattr(seed_fd, "user.random-seed-creditable", "1", 1, 0) < 0)
 304                         log_full_errno(ERRNO_IS_NOT_SUPPORTED(errno) ? LOG_DEBUG : LOG_WARNING, errno,
 305                                        "Failed to mark seed file as creditable, ignoring: %m");
 306         return 0;
 307 }
 308
 309 static int help(int argc, char *argv[], void *userdata) {
 310         _cleanup_free_ char *link = NULL;
 311         int r;
 312
 313         r = terminal_urlify_man("systemd-random-seed", "8", &link);
 314         if (r < 0)
 315                 return log_oom();
 316
 317         printf("%1$s [OPTIONS...] COMMAND\n"
 318                "\n%5$sLoad and save the system random seed at boot and shutdown.%6$s\n"
 319                "\n%3$sCommands:%4$s\n"
 320                "  load                Load a random seed saved on disk into the kernel entropy pool\n"
 321                "  save                Save a new random seed on disk\n"
 322                "\n%3$sOptions:%4$s\n"
 323                "  -h --help           Show this help\n"
 324                "     --version        Show package version\n"
 325                "\nSee the %2$s for details.\n",
 326                program_invocation_short_name,
 327                link,
 328                ansi_underline(),
 329                ansi_normal(),
 330                ansi_highlight(),
 331                ansi_normal());
 332
 333         return 0;
 334 }
 335
 336 static const char* const seed_action_table[_ACTION_MAX] = {
 337         [ACTION_LOAD] = "load",
 338         [ACTION_SAVE] = "save",
 339 };
 340
 341 DEFINE_PRIVATE_STRING_TABLE_LOOKUP_FROM_STRING(seed_action, SeedAction);
 342
 343 static int parse_argv(int argc, char *argv[]) {
 344         enum {
 345                 ARG_VERSION = 0x100,
 346         };
 347
 348         static const struct option options[] = {
 349                 { "help",    no_argument, NULL, 'h'         },
 350                 { "version", no_argument, NULL, ARG_VERSION },
 351         };
 352
 353         int c;
 354
 355         assert(argc >= 0);
 356         assert(argv);
 357
 358         while ((c = getopt_long(argc, argv, "h", options, NULL)) >= 0)
 359                 switch (c) {
 360                 case 'h':
 361                         return help(0, NULL, NULL);
 362                 case ARG_VERSION:
 363                         return version();
 364                 case '?':
 365                         return -EINVAL;
 366
 367                 default:
 368                         assert_not_reached();
 369                 }
 370
 371         if (optind + 1 != argc)
 372                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "This program requires one argument.");
 373
 374         arg_action = seed_action_from_string(argv[optind]);
 375         if (arg_action < 0)
 376                 return log_error_errno(arg_action, "Unknown action '%s'", argv[optind]);
 377
 378         return 1;
 379 }
 380
 381 static int run(int argc, char *argv[]) {
 382         _cleanup_free_ struct sha256_ctx *hash_state = NULL;
 383         _cleanup_close_ int seed_fd = -1, random_fd = -1;
 384         bool read_seed_file, write_seed_file, synchronous;
 385         size_t seed_size;
 386         int r;
 387
 388         log_setup();
 389
 390         r = parse_argv(argc, argv);
 391         if (r <= 0)
 392                 return r;
 393
 394         umask(0022);
 395
 396         r = mkdir_parents(RANDOM_SEED, 0755);
 397         if (r < 0)
 398                 return log_error_errno(r, "Failed to create directory " RANDOM_SEED_DIR ": %m");
 399
 400         /* When we load the seed we read it and write it to the device and then immediately update the saved
 401          * seed with new data, to make sure the next boot gets seeded differently. */
 402
 403         switch (arg_action) {
 404         case ACTION_LOAD:
 405                 seed_fd = open(RANDOM_SEED, O_RDWR|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
 406                 if (seed_fd < 0) {
 407                         int open_rw_error = -errno;
 408
 409                         write_seed_file = false;
 410
 411                         seed_fd = open(RANDOM_SEED, O_RDONLY|O_CLOEXEC|O_NOCTTY);
 412                         if (seed_fd < 0) {
 413                                 bool missing = errno == ENOENT;
 414                                 int level = missing ? LOG_DEBUG : LOG_ERR;
 415
 416                                 log_full_errno(level, open_rw_error, "Failed to open " RANDOM_SEED " for writing: %m");
 417                                 log_full_errno(level, errno, "Failed to open " RANDOM_SEED " for reading: %m");
 418
 419                                 return missing ? 0 : -errno;
 420                         }
 421                 } else
 422                         write_seed_file = true;
 423
 424                 random_fd = open("/dev/urandom", O_RDWR|O_CLOEXEC|O_NOCTTY);
 425                 if (random_fd < 0)
 426                         return log_error_errno(errno, "Failed to open /dev/urandom: %m");
 427
 428                 read_seed_file = true;
 429                 synchronous = true; /* make this invocation a synchronous barrier for random pool initialization */
 430                 break;
 431
 432         case ACTION_SAVE:
 433                 random_fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
 434                 if (random_fd < 0)
 435                         return log_error_errno(errno, "Failed to open /dev/urandom: %m");
 436
 437                 seed_fd = open(RANDOM_SEED, O_WRONLY|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
 438                 if (seed_fd < 0)
 439                         return log_error_errno(errno, "Failed to open " RANDOM_SEED ": %m");
 440
 441                 read_seed_file = false;
 442                 write_seed_file = true;
 443                 synchronous = false;
 444                 break;
 445
 446         default:
 447                 assert_not_reached();
 448         }
 449
 450         r = random_seed_size(seed_fd, &seed_size);
 451         if (r < 0)
 452                 return r;
 453
 454         if (read_seed_file)
 455                 r = load_seed_file(seed_fd, random_fd, seed_size,
 456                                    write_seed_file ? &hash_state : NULL);
 457
 458         if (r >= 0 && write_seed_file)
 459                 r = save_seed_file(seed_fd, random_fd, seed_size, synchronous, hash_state);
 460
 461         return r;
 462 }
 463
 464 DEFINE_MAIN_FUNCTION(run);